Investigation of carbon flux and sulphide oxidation kinetics during passive biotreatment of mine water

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The impact of acid rock drainage (ARD) on South Africa’s already threatened water resources is a serious concern. While acid waters emanating from groundwater rebound through the Witwatersrand gold basins has received the majority of the media attention and elicited the strongest response from the authorities, ARD from diffuse sources, primarily associated with coal mining, is likely to impact a far larger area. The traditional chemical and physical interventions are not particularly well suited to these discharges. Research into passive and semi-passive systems has met with varying degrees of success. Typically, the processes that target sulphate salinity make use of biological sulphate reduction, often utilising complex organic carbon sources to provide the electron donor. The sulphide product is highly toxic and presents a significant risk to the environment and human health and needs to be carefully managed. The most attractive option is the partial oxidation of sulphide to elemental sulphur, which is stable and has commercial value.
The primary aim of the research was to characterise the carbon flux through an integrated sulphate reduction/sulphide oxidation process and determine its effect on the recovery of sulphur in the floating sulphur biofilm. The second aim was to investigate oxygen mass transfer to the biofilm and use this information to inform optimal management of the system.
Two packed bed columns were used to investigate the sulphate reduction efficiency and carbon flux. A series of linear flow channel reactors (LFCRs) were used to investigate the effect or residence time and acetate supplementation on sulphide oxidation rate and elemental sulphur yield. The oxygen mass transfer into the biofilm was investigated in a scaled-down reactor, using a dissolved oxygen microprobe. The composition and internal structure of the floating sulphur biofilm was analysed using scanning electron microscopy and elemental analysis. The structure of the biofilm informed the interpretation of the data.
The data allowed the specific aims to be addressed and provided significant insights into the performance of the integrated system. The experiments showed that efficient sulphide oxidation was possible within the floating sulphur biofilm in the LFCR, provided the feed was supplemented with organic carbon. A hydraulic residence time between one and two days was optimal. In order to sustain optimal performance the biofilm would need to be harvested every two to three residence times. Under these condition sulphide oxidation rates of up to 5.5 mmol/ℓ.day could be achieved, with at least 75% of the oxidised sulphide reporting to the biofilm as elemental sulphur. Conservatively, this represents a sulphur recovery of 13.5 g/m2.day, for the current reactor configuration.
The experiments illustrated that organic carbon liberation from packed bed reactors is unlikely to be sufficient to sustain efficient levels of sulphate reduction beyond the short term, once the readily labile organic carbon has been liberated. Supplementation with relatively significant (1 g/ℓ) concentrations of readily usable organic carbon, such as acetate, was needed to sustain sulphate reduction. While the majority of the sulphate reduction (± 75%) was reliant on the acetate, continued hydrolysis of the lignocellulose was observed. Despite this, the VFA concentration in the effluent from the packed bed reactors was negligible after the first four months. Therefore, further organic carbon supplementation (> 100 mg/ℓ acetate) of the feed to the LFCR was necessary for biofilm development and efficient sulphide oxidation.
Under optimal conditions the biofilm formed within 12 hours, following which the oxygen mass transfer into the liquid was significantly reduced (kf from 1.65 × 10-4 m/s to 2.35 × 10 6 m/s between 24 and 48 hours). The reduced mass transfer prevented complete sulphide oxidation, so the majority of the sulphide was oxidised to sulphur within the biofilm. The HRT and sulphide loading affected the rate of formation and structure of the biofilm, influencing performance. Optimal performance was achieved at an HRT between one and two days. Harvesting of the biofilm would be required every two to three residence times to maintain optimum sulphide oxidation rates.